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Project 1C: Deque61B Enhancements

Due: Tuesday, February 20th, 11:59 PM PT


Each assignment will have an FAQ linked at the top. You can also access it by adding “/faq” to the end of the URL. The FAQ for Project 1C is located here.


In Project 1A, we built LinkedListDeque61B and in Project 1B, we built ArrayDeque61B. Now we’ll see a different implementation: MaxArrayDeque61B! This part of the project will provide some enhancements to your previous ArrayDeque61B and LinkedListDeque61B, and also bring everything together into an application of your newly-built data structure.

By the end of Project 1C, you will complete the following:

  • Write the iterator(), equals(), and toString() methods for and
  • Implement
  • Finish the GuitarHero tasks.

This section assumes you have watched and fully digested the lectures up till the Iterators, Object Methods lecture, Lecture 11.


As in Project 1B, we will be enforcing style. You must follow the style guide, or you will be penalized on the autograder.

You can and should check your style locally with the CS 61B plugin. We will not remove the velocity limit for failing to check style.

Getting the Skeleton Files

Follow the instructions in the Assignment Workflow guide to get the skeleton code and open it in IntelliJ. For this project, we will be working in the proj1c directory.

You see a proj1c directory appear in your repo with the following structure:

├── src
│   ├── deque
│   │   ├──
│   │   ├──
│   │   └──
│   └──gh2
│       ├──
│       ├──
│       ├──
│       └──
└── tests

If you get some sort of error, STOP and either figure it out by carefully reading the git WTFs or seek help at OH or Ed. You’ll potentially save yourself a lot of trouble vs. guess-and-check with git commands. If you find yourself trying to use commands recommended by Google like force push, don’t.

Don’t use force push, even if a post you found on Stack Overflow says to do it!

You can also watch Professor Hug’s demo about how to get started and this video if you encounter some git issues.

Object Methods

If you’d like, you can follow the steps in this short video guide to help you get set up for Project 1C!

In order to implement the following methods, you should start by copying and pasting your Project 1A and Project 1B implementations of LinkedListDeque61B and ArrayDeque61B into the relevant files in your proj1c directory.

Please keep package deque; at the top of both files. Otherwise, your code will not compile.


One shortcoming of our Deque61B interface is that it can not be iterated over. That is, the code below fails to compile with the error “foreach not applicable to type”.

  Deque61B<String> lld1 = new LinkedListDeque61B<>();

  for (String s : lld1) {

Similarly, if we try to write a test that our Deque61B contains a specific set of items, we’ll also get a compile error, in this case: “Cannot resolve method containsExactly in Subject”.

public void addLastTestBasicWithoutToList() {
    Deque61B<String> lld1 = new LinkedListDeque61B<>();

    lld1.addLast("front"); // after this call we expect: ["front"]
    lld1.addLast("middle"); // after this call we expect: ["front", "middle"]
    lld1.addLast("back"); // after this call we expect: ["front", "middle", "back"]
    assertThat(lld1).containsExactly("front", "middle", "back");

Again the issue is that our item cannot be iterated over. The Truth library works by iterating over our object (as in the first example), but our LinkedListDeque61B does not support iteration.

To fix this, you should first modify the Deque61B interface so that the declaration reads:

public interface Deque61B<T> extends Iterable<T> {

Next, implement the iterator() method using the techniques described in lecture 11.

Task: Implement the iterator() method in both LinkedListDeque61B and ArrayDeque61B according to lecture.

You are not allowed to call toList here.


Consider the following code:

    public void testEqualDeques61B() {
        Deque61B<String> lld1 = new LinkedListDeque61B<>();
        Deque61B<String> lld2 = new LinkedListDeque61B<>();




If we run this code, we see that we fail the test, with the following message:

expected: [front, middle, back]
but was : (non-equal instance of same class with same string representation)

The issue is that the Truth library is using the equals method of the LinkedListDeque61B class. The default implementation is given by the code below:

    public boolean equals(Object obj) {
        return (this == obj);

That is, the equals method simply checks to see if the addresses of the two objects are the same. We want to be able to check whether the two Deque61B objects are equal in terms of elements and order so therefore we need a different equals method.

Override the equals method in the ArrayDeque61B and LinkedListDeque61B classes. For guidance on writing an equals method, see the lecture slides or the lecture code repository.

Note: You might ask why we’re implementing the same method in two classes rather than providing a default method in the Deque61B interface. Interfaces are not allowed to provide default methods that override Object methods. For more see

However, one workaround for this is to provide a default, non-Object helper method in the Deque61B interface and have the implementing classes call it.

Override the equals() method in the LinkedListDeque61B and ArrayDeque61B classes.

Important: You should not use getClass, and there’s no need to do any casting in your equals method. That is, you shouldn’t be doing (ArrayDeque61B) o. Such equals methods are old fashioned and overly complex. Use instanceof instead.

Note: The instanceof operator behaves a little strangely with generic types, for reasons beyond the scope of this course. For example, if you want to check if lst is an instance of a List<Integer>, you should use lst instanceof List<?> rather than lst instanceof List<Integer>. Unfortunately, this is not able to check the types of the elements, but it’s the best we can do.

Important: Make sure you use the @Override tag when overriding methods. A common mistake in student code is to try to override equals(ArrayList<T> other) rather than equals(Object other). Using the optional @Override tag will prevent your code from compiling if you make this mistake. @Override is a great safety net.

You are not allowed to call toList here.


Consider the code below, which prints out a LinkedListDeque61B.

Deque61B<String> lld1 = new LinkedListDeque61B<>();



This code outputs something like deque.proj1a.LinkedListDeque61B@1a04f701. This is because the print statement implicitly calls the LinkedListDeque61B toString method. Since you didn’t override this method, it uses the default, which is given by the code below (you don’t need to understand how this code works).

    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());

In turn the hashCode method, which you have also not overridden, simply returns the address of the object, which in the example above was 1a04f701.

Task: Override the toString() method in the LinkedListDeque61B and ArrayDeque61B classes, such that the code above prints out [front, middle, back].

Hint: Java’s implementation of the List interface has a toString method.

Hint: There is a one line solution (see hint 1).

Hint: Your implementation for LinkedListDeque61B and ArrayDeque61B should be exactly the same.

Testing The Object Methods

We haven’t provided you with test files for these three object methods; however, we strongly encourage you to use the techniques you learned from projects 1A and 1B to write your own tests. You can structure these tests however you’d like, since we won’t be testing them. One possible (and suggested) structure is to create two new files in the tests directory called LinkedListDeque61BTest and ArrayDeque61BTest, similar to the ones we gave you in 1A and 1B.


After you’ve fully implemented your ArrayDeque61B and tested its correctness, you will now build the MaxArrayDeque61B. A MaxArrayDeque61B has all the methods that an ArrayDeque61B has, but it also has 2 additional methods and a new constructor:

  • public MaxArrayDeque61B(Comparator<T> c): creates a MaxArrayDeque61B with the given Comparator. (You may import java.util.Comparator for this.)
  • public T max(): returns the maximum element in the deque as governed by the previously given Comparator. If the MaxArrayDeque61B is empty, simply return null.
  • public T max(Comparator<T> c): returns the maximum element in the deque as governed by the parameter Comparator c. If the MaxArrayDeque61B is empty, simply return null.

The MaxArrayDeque61B can either tell you the max element in itself by using the Comparator<T> given to it in the constructor, or an arbitrary Comparator<T> that is different from the one given in the constructor.

We do not care about the equals(Object o) method of this class, so feel free to define it however you think is most appropriate. We will not test this method.

For testing, you can use Comparator.naturalOrder() in your own test files. This Comparator is using naturalOrder(). If your generic type is Integer, you can create your MaxArrayDeque61B using the following example:

MaxArrayDeque61B<Integer> m = new MaxArrayDeque61B<Integer>(Comparator.naturalOrder());

If you find yourself starting off by copying your entire ArrayDeque61B implementation in a MaxArrayDeque61B file, then you’re not doing this assignment in the intended manner. This is an exercise in clean code, and redundancy is one our worst enemies when battling complexity! For a hint, re-read the second sentence of this section above.

Task: Fill out the file according to the API above.

There are no runtime requirements on these additional methods, we only care about the correctness of your answer. Sometimes, there might be multiple elements in the MaxArrayDeque61B that are all equal and hence all the max: in in this case, you can return any of them and they will be considered correct.

You should write tests for this part as well! You’ll likely be creating multiple Comparator<T> classes to test your code: this is the point! To get practice using Comparator objects to do something useful (find the maximum element) and to get practice writing your own Comparator classes. You will not be turning in these tests, but we still highly suggest making them for your sake.

You will not use the MaxArrayDeque61B you made for the next part; it’ll be in an isolated exercise.

Guitar Hero

In this part of the project, we will create another package for generating synthesized musical instruments using the deque package we just made. We’ll get the opportunity to use our data structure for implementing an algorithm that allows us to simulate the plucking of a guitar string.

The GH2 Package

The gh2 package has just one primary component that you will edit:

  • GuitarString, a class which uses an Deque61B<Double> to implement the Karplus-Strong algorithm to synthesize a guitar string sound.

We’ve provided you with skeleton code for GuitarString which is where you will use your deque package that you made in the first part of this project.


We want to finish the GuitarString file, which should use the deque package to replicate the sound of a plucked string. Note that this file uses the word “buffer”, which is a synonym for “deque” in this context.

We’ll be using the Karplus-Strong algorithm, which is quite easy to implement with a Deque61B. It is simply the following three steps:

  1. Replace every item in a Deque61B with random noise (double values between -0.5 and 0.5).
  2. Play the double at the front of the Deque61B.
  3. Remove the front double in the Deque61B and average it with the next double in the Deque61B (hint: use removeFirst) and get()) multiplied by an energy decay factor of 0.996 (we’ll call this entire quantity newDouble). Then, add newDouble to the back of the Deque61B. Go back to step 2 (and repeat forever).

Or visually, if the Deque61B is as shown on the top, we’d play the 0.2, remove it, combine it with the 0.4 to form 0.2988, and add the 0.2988.


You can play a double value with the method. For example will tell the diaphragm of your speaker to extend itself to 1/3rd of its total reach, will tell it to stretch its little heart backwards almost as far as it can reach. Movement of the speaker diaphragm displaces air, and if you displace air in nice patterns, these disruptions will be interpreted by your consciousness as pleasing thanks to billions of years of evolution. See this page for more. If you simply do and never play anything again, the diaphragm shown in the image would just be sitting still 9/10ths of the way forwards.

Complete so that it implements the Karplus-Strong algorithm. Note that you will have to fill your Deque61B buffer with zeros in the GuitarString constructor. Part of the process will be handled by the client of the GuitarString class. You are only required to complete the tasks labeled with TODO.

Do not call in This will cause the autograder to break. does this for you already.

Make sure to add the libraries, as usual, otherwise IntelliJ won’t be able to find StdAudio.

For example, the provided TestGuitarString class provides a sample test testPluckTheAString that attempts to play an A-note on a guitar string. If you run the test should hear an A-note when you run this test. If you don’t, you should try the testTic method and debug from there. Consider adding a print or toString method to that will help you see what’s going on between tics.

Note: we’ve said Deque61B here, but not specified which Deque61B implementation to use. That is because we only need those operations addLast, removeFirst, and get and we know that classes that implement Deque61B have them. So you are free to choose either the LinkedListDeque61B for the actual implementation, or the ArrayDeque61B. For an optional (but highly suggested) exercise, think about the tradeoffs with using one vs the other and discuss with your friends what you think the better choice is, or if they’re both equally fine choices.

Why It Works

The two primary components that make the Karplus-Strong algorithm work are the ring buffer feedback mechanism and the averaging operation.

  • The ring buffer feedback mechanism. The ring buffer models the medium (a string tied down at both ends) in which the energy travels back and forth. The length of the ring buffer determines the fundamental frequency of the resulting sound. Sonically, the feedback mechanism reinforces only the fundamental frequency and its harmonics (frequencies at integer multiples of the fundamental). The energy decay factor (.996 in this case) models the slight dissipation in energy as the wave makes a round trip through the string.
  • The averaging operation. The averaging operation serves as a gentle low-pass filter (which removes higher frequencies while allowing lower frequencies to pass, hence the name). Because it is in the path of the feedback, this has the effect of gradually attenuating the higher harmonics while keeping the lower ones, which corresponds closely with how a plucked guitar string sounds.


You should now also be able to use the GuitarHeroLite class. Running it will provide a graphical interface, allowing the user (you!) to interactively play sounds using the gh2 package’s GuitarString class.


To submit the project, add and commit your files, then push to your remote repository. Then, go to the relevant assignment on Gradescope and submit there.

The autograder for this assignment will have the following velocity limiting scheme:

  • From the release of the project to the due date, you will have 4 tokens; each of these tokens will refresh every 24 hours.


This project, similar to Project 0, is divided into individual components, each of which you must implement completely correctly to receive credit.

  1. LinkedListDeque61B Object Methods (20%): Correctly implement iterator, equals, and toString in LinkedListDeque61B.
  2. ArrayDeque61B Object Methods (20%): Correctly implement iterator, equals, and toString in ArrayDeque61B.
  3. MaxArrayDeque61B Functionality (5%): Ensure your MaxArrayDeque61B correctly runs all the methods in the Deque61B interface.
  4. MaxArrayDeque61B Max (35%): Correctly implement max in MaxArrayDeque61B.
  5. GuitarString (20%): Correctly implement the GuitarString client class.

In total, Project 1c is worth 10 points.